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conversion that only relies on the ambient spontaneous moisture diffusion pro-cess greatly avoids the disturbances (e.g., thermal, mechanical variation, and so on) from the environment, which is of sig-nificance for the future high stable electric power supply. Therefore, to further satisfy the increasing demands for electric power, the effort on exploring and researching new versatile candidates (not just limited in a single less-conductive material, such as GO) for various vapor-electric energy generation is urgent, which has been absent so far yet.

Conducting polymers have attracted numerous attention in the applications

of actuators, sensors, batteries, supercapacitors, and electro-chromic devices due to their predominant mechanical, optical, redox, and electrical properties.[12–15] In particular, polypyr-role (PPy) has been widely studied in energy conversion fields because of its significant advantages of easy synthesis, wide range of dopant species, and relatively good environmental stability.[16] For instance, PPys can act as a kind of hygroscopic material to capture the moisture from the ambient environ-ment,[15,17,18] where its dominant ionic conductivity enhances with the increase of relative humidity (RH). Furthermore, PPys could promote the conversion of electrical to mechan-ical energy in the form of actuation behaviors induced by the volume change resulting from a Faradaic doping and undoping process.[19] Meanwhile, the doped and undoped anions lead to different conducting states of PPy, in which the energy gap reduces from 4 to ≤2.5 eV according to the state from undoped (neutral) to doped (oxidation).[20,21] The reversible switching process between the doped (oxidized) state and undoped (neu-tral) state of PPy can be controlled by changing the electrical potential as well as the doping level.[22] These results indicate the strong interactions of PPy and doped anions to tune the functions of PPy-based devices that provide the opportunities for developing new generation energy-converting systems.

In this work, we have developed a high-performance vapor-activated power generator (VaPG) based on a 3D PPy frame-work with a preformed anion gradient (named as gradient 3D PPy, g-3D-PPy). The g-3D-PPy was formed by electrolyte/elec-tric field coinduced by the gradient process of anions doped in the 3D PPy framework containing LiClO4 aqueous solu-tion (this strategy here is called electrolyte-electric annealing, EeA process).[10,22] Upon exposure to the water vapor, the open porous network can greatly facilitate the diffusion of water molecules, and meanwhile the anion-containing gradient pro-vides free ionic gradient to promote the spontaneous transport

Vapor-Activated Power Generation on Conductive Polymer

Jiangli Xue, Fei Zhao, Chuangang Hu, Yang Zhao,* Hongxia Luo, Liming Dai, and Liangti Qu*

An efficient vapor-activated power generator based on a 3D polypyrrole (PPy) framework was demonstrated for the first time. By constructing the anions gradient in the PPy, this specially designed PPy framework provided free ionic gradient with the assistant of absorbing water vapor to promote the spontaneous transport of ionic charge carriers, thus leading to the intermit-tent electric output with the change of external water vapor. A high voltage output of ≈60 mV and power density output of ≈6.9 mW m−2 were achieved under the moisture environment. More interestingly, it also exhibited power generation behaviors upon exposure to most of organic or inorganic vapors, indicating the potential new type of self-powered vapor sensors for practical applications.

DOI: 10.1002/adfm.201604188

Dr. J. Xue, Dr. F. Zhao, Dr. Y. Zhao, Prof. L. QuBeijing Key Laboratory of Photoelectronic/ Electrophotonic Conversion MaterialsKey Laboratory of Cluster ScienceMinistry of Education of ChinaSchool of ChemistryBeijing Institute of TechnologyBeijing 100081, P. R. ChinaE-mail: yzhao@bit.edu.cn; lqu@bit.edu.cnDr. C. Hu, Prof. L. DaiCenter of Advanced Science and Engineering for Carbon (Case 4Carbon)Department of Macromolecular Science and EngineeringCase School of EngineeringCase Western Reserve UniversityCleveland, OH 44106, USAProf. H. LuoDepartment of ChemistryRenmin University of ChinaBeijing 100872, P. R. China

1. Introduction

Electric power, as a typical clean and renewable energy, plays an important role in the modern society with the emerging new types of electronics, such as E-newsletter and electric vehi-cles and so on. Recently, ongoing efforts in the development of electric power generators have led to the birth of various novel devices based on harvesting and transforming energy from environment into electricity, including thermoelectric,[1–3] piezoelectric,[4–6] and triboelectric[7–9] generator. Remark-ably, we recently proposed a graphene-based moisture-acti-vated power generator (MaPG) which can directly convert the chemical potential energy derived from moisture diffusion to electric power by establishing the oxygen group gradient in gra-phene oxide (GO).[10,11] Such ideal and high efficiency energy

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of ionic charge carriers, leading to the intermittent electric output with the change of external water vapor. The as-prepared g-3D-PPy is able to provide a voltage output of ≈60 mV and a power density of ≈6.9 mW m−2, which even surpass the elec-tric outputs of MaPG based on GO film we reported recently.[10] More interestingly, it also exhibits power generation behaviors upon exposure to most of organic or inorganic vapors, indi-cating the potential new type of self-powered vapor sensors for practical applications.

2. Results and Discussion

Figure 1a illustrates the preparation process of 3D PPy by using a template-assisted electrochemical polymerization method. As an initial template, the vanadium pentoxide (V2O5) foam fabricated via a simple hydrothermal process[23] was directly immersed into the ethanol solution containing 10 vol% Py fol-lowed by solvothermal treatment in a sealed Teflon-lined auto-clave at 190 °C for several hours to obtain the pyrrole (Py)-V2O5 foam (Figure S1, Supporting Information).[23–25] For prepara-tion of PPy-V2O5 foam, the Py-V2O5 foam acted as the working

electrode in a three-electrode cell with a Pt sheet as counter electrode and Ag/AgCl as reference electrode, respectively. The PPy-V2O5 foam was then formed through electrochemical polymerization of Py monomer adhered on V2O5 sheets in 0.2 m LiClO4 ethanol solution by applying a constant potential of 0.8 V.[24] After removing the V2O5 nanosheets by HCl, the 3D PPy was finally obtained.

The as-prepared 3D PPy possesses a low density of ≈10 mg cm−3, which is almost seven times lower than that of PPy sponges (70 mg cm−3),[26] comparable to that of graphene aerogel (≈10 mg cm−3)[27] and graphite foam (9.5 mg cm−3).[28] As shown in Figure 1b, a 1.3 cm3 PPy foam stands stably on the top of a dandelion without any structure deformation of the dandelion. The scanning electron microscopy (SEM) image shows the PPy foam consists of a neat interconnected 3D network with randomly open pore structures (Figure 1c).[29] A closer look in Figure 1d reveals a smooth and clean surface of the PPy sheet, indicating the totally removal of the residual V2O5, which is also confirmed by X-ray energy dispersive spec-troscopy (EDS) spectrum (Figure S2, Supporting Information). Moreover, the walls possess a few PPy layers with a thickness of ≈10 nm (Figure 1d, the insert).[8] Raman spectroscopic

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Figure 1. a) Schematics of the fabrication process of the 3D PPy framework. b) Digital photo of 3D PPy foam standing on a dandelion. c) Typical SEM image of the 3D PPy framework architecture constructed from nanosheets. d) The corresponding magnified view of (c). Inset in panel (d) is the enlarged view of the edge wall of 3D PPy framework. e,f) Raman spectrum and the nitrogen adsorption isotherm of 3D PPy framework, respectively. Scale bars: b) 1 cm, c) 10 µm, d) 10 µm, inset in (d) 1 µm.

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measurements exhibit a series of typical peaks at ≈880, 930, 1050, 1238, 1370, 1410, and 1590 cm−1 which are consistent with those of electropolymerized PPy, further confirming the successful formation of PPy (Figure 1e).[30,31] The Brunauer–Emmett–Teller (BET) nitrogen adsorption isotherm dem-onstrates a specific surface area of approximately 100 m2 g−1 (Figure 1f) suggesting the predominant macroporous structure of the 3D PPy in consistence with SEM observation (Figure 1c).

It is believed that PPy can be doped with anions (such as ClO4

−) through the electrochemical polymerization process in an LiClO4 aqueous solution.[32] When applying a positive poten-tial, the ClO4

− tends to implant into the PPy molecule skeleton. The ClO4

− releases from PPy when the potential is switched to negative potential,[33,34] which can be illustrated in the following reaction equation[34]

PPy (ClO ) e PPy ClO40

4+ + → +− − −

(1)

The above reaction process offers the possibility to construct a novel g-3D-PPy framework with gradient distribution of ClO4

−. Consequently, in order to form the uniform ClO4− gra-

dient, the 3D PPy containing LiClO4 aqueous solution was sand-wiched between two pieces of gold (Au) electrodes connecting with external circuit in an enclosed container (Figure 2a,b; Figure S3, Supporting Information). A bias voltage of 3 V was subsequently applied on the two sides of the 3D PPy (Figure S4, Supporting Information). During the EeA process, the side of 3D PPy connected with positive electrode was oxidized accom-panied by ClO4

− uptaking process (noted as bottom side), while the opposite side was reduced naturally accompanied by ClO4

− releasing process (noted as top side), which results in the move-ment of ClO4

− followed the electron (e−) flow (Figure 2b). After freeze-drying treatment, the as-formed g-3D-PPy foam was then compressed into a tablet to further improve the physical contact of PPy sheets, enhance the gradient distribution of ClO4

−, and facilitate the ion transport (Figure 2c,d).[30,35]

Figure 2e shows the cross section SEM image of the g-3D-PPy tablet with a thickness of ≈100 µm, where the size of tablet can be controlled with demands by simple cutting the sample. The cross-sectional compositional of g-3D-PPy is verified by line scan of SEM–EDS (Figure 2f,g). As expected, different from the evenly distribution of carbonaceous component, the uni-form chlorine gradient distribution can be easily distinguished in Figure 2g, in which the intensity of chlorine fades away from bottom to top side, indicating the successful formation of ClO4

− gradient in g-3D-PPy. The chemical status of C, Cl, and O elements for g-3D-PPy is also investigated by the X-ray photo-electron spectroscopy (XPS). As shown in Figure 2h, the Cl/C atomic ratio on the bottom side of g-3D-PPy is ≈0.57%, which is higher than the original 3D PPy (≈0.29%) and about four times the top side of g-3D-PPy (≈0.15%). Moreover, the high resolu-tion Cl 2p spectra (Figure S5, Supporting Information) reveals a typical ClO4

− related peak at ≈210 eV after applying a bias of 3 V, further indicating the excellent ability for g-3D-PPy to capture ClO4

− . Apart from the peaks for C, Cl, and O elements, both of top and bottom sides exhibit the pronounced N 1s peaks. As shown in Figure 2i, the two typical peaks at ≈400 and 401 eV belong to the pyrrolyl nitrogen (NH structure) and the posi-tively charged nitrogen (NH+ structure), respectively.[36–39]

In order to quantitatively measure the response to ambient humidity, the as-formed g-3D-PPy is sandwiched by two pieces of Au electrodes and then connected to the test circuit within an enclosed RH controlling system (Figure S6, Supporting Information). For comparison, the original PPy film prepared by directly electrochemical polymerization of Py monomer at the same condition (Figure S7, Supporting Information) is also treated under the same EeA process. As demonstrated in Figure 3a,b, the VaPG based on g-3D-PPy can provide stable voltage and current output of ≈60 mV and ≈10 µA cm−2 under a ΔRH of 85%, which are far larger than the electrochemical polymerized PPy film with a voltage and current output of ≈0.02 mV and ≈0.2 µA cm−2 (Figure S8, Supporting Informa-tion), and even two times that of previous reported GO film with a preformed oxygen-containing group gradient,[10] similar to the porous carbon films,[40] indicating the efficient mois-ture responsive capability for the g-3D-PPy. In contrast, the g-3D-PPy without tableting only provides much small voltage and current output of ≈4 mV and ≈0.2 µA cm−2 (Figure S9, Supporting Information), probably due to the unconspicuous of ClO4

− gradient makes the water molecules difficult to dis-sociate enough free ions in a very short time and transport ions or charges in the long transmission pathway of macroporous polymer skeleton during dehydration process.

Notably, similar to our previous work,[10] an asymmetric behavior of both voltage and current output pulses for g-3D-PPy is also observed in Figure 3c,d, in which the positive pulse peak value is much higher than that of the negative pulse. Unlike the conventional electric generators, this unique electric output is consistent with the hydration and dehydration process. The induced positive voltage or current rapidly increases in the hydration process, and goes back to the initial state once removing the water. Owing to the faster falling speed of elec-tric pulses than withdrawing of water, the electric pulses even cross the zero and reach to the negative value for a few seconds until the environmental water content falls back to the initial value in dehydration process. Furthermore, the typical electric output exhibits stable signal for 200 cycles without any decay (Figure 3e,f). Figure 3g shows the electric pulse as function of different ΔRH, in which both the voltage and current outputs are enhanced as the increase of ΔRH. The maximum power density output is controlled by the maximum open-circuit voltage (Vmax) and the maximum short-circuit current density (Jmax), which can be calculated as[11]

max max maxP V J= ⋅ (2)

Considering the relationship of power density output and ΔRH, the calculated maximum output power density can be achieved to about 6.9 mW m−2 under the ΔRH of ≈85% (Figure 3h). The detailed voltage or current behaviors of VaPG based on g-3D-PPy at different RH are shown in Figure S10 (Supporting Information).

Apart from water as an energy source, it is interesting that the ethanol is also able to activate the power generation of g-3D-PPy. As one of common polar solvents, ethanol is free to go in and out of the porous structure of g-3D-PPy, which provides an opportunity for construct ethanol-driving self-powered sensor in the air. To evaluate the response to ethanol

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vapor, a simple testing set-up is constructed (Figure S11, Sup-porting Information). Both the voltage and current output showed a significant response signals upon exposure to 0–250 ppm of ethanol vapor diluted with N2 (Figure 4a,b). Similar to the moisture-induced electric generation behavior, the g-3D-PPy provides stable voltage output of ≈28 mV and current density of ≈2.3 µA cm−2 under the 250 ppm of ethanol vapor (Figure 4a,c). In contrast, no any voltage and current output signals of the g-3D-PPy can be observed when exposed to pure N2 (Figure S12, Supporting Information). Moreover, the device performance is only mainly subject to the con-centration of ethanol vapor. As demonstrated in Figure 4b,d,

both the voltage and current density output can be enhanced with the increase of ethanol vapor concentration, revealing a linear relationship between ethanol concentration and voltage or current density. The very high slopes of the voltage and current response curves of g-3D-PPy indicate an excel-lent sensitivity to ethanol.[41] Impressively, we found that the electric generation behavior of g-3D-PPy can also be inspired by other organic and inorganic solvent vapors, including methanol, acetone, NH3⋅H2O, and HCl⋅nH2O, in which the output of voltage and current trailed off as the polarity abate of the organic solvent (Figure 4e,f). It is believed that the ionization ability of LiClO4 in solvent mainly depends on the

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Figure 2. Preparation process and characterizations of the g-3D-PPy. a–d) The EeA process of PPy with an applied voltage of 3 V for 500 s. e) Cross-section SEM image of the g-3D-PPy tablet. f,g) The corresponding carbon and chlorine linear scan EDS spectrum of (e). h) XPS spectra on top and bottom sides of g-3D-PPy. i) The corresponding high resolution N 1s peak of top and bottom sides of g-3D-PPy.

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dielectric constant of the solvent. The greater dielectric con-stant causes the stronger solvent polarity, resulting in dis-sociating much more free ions and thus leading to a higher voltage and current output.[42] Usually, the strength of solvent polarity decreases in the order of methanol, ethanol, and ace-tone, indicating that the number of free ions is reduced with the decrease of solvent polarity which gives rise to a varying electric voltage and current density in different solvents.[43,44]

The output of current response to NH3⋅H2O is lowest among these vapors, probably due to the difficult desorption of NH3 molecules in PPy.[39,45,46] Based on these above findings, this self-powered sensor can be widely applied for detecting organic vapor or acid/alkaline molecules. For comparison, the vapor-activated power generators based on graphene and other related materials are also listed in Table S1 of the Sup-porting Information.

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Figure 3. Experimental data of a) voltage and b) current output cycles of the g-3D-PPy in response to the periodic RH variation (ΔRH = 85%). One of single output cycles of c) voltage and d) current. The stability cycling tests of e) voltage and f) current output for g-3D-PPy in response to the periodic RH variation (ΔRH = 85%). The curves of g) voltage, current output, and h) the corresponding power output in response to the various ΔRH.

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Usually, the freshly electropolymerized 3D PPy is doped with electrolyte anions in the oxidized state. The following reactions may occur during redox cycling of PPy[12,47]

PPy A e PPy A0+ → ++ − − − (3)

PPy A M e PPy A M0+ + →+ − + − − + (4)

PPy A PPy A e0 + → +− + − − (5)

PPy A M PPy A M e0 → + +− + + − + − (6)

PPy A M PPy A M0 0→ + +− + − + (7)

PPy+A− represents the PPy doped with mobile anion A− (such as ClO4

−) indicating the oxidized state of PPy, and PPy0 is the reduced state. The reduction of PPy+A− process results in the removing of the A− from polymer (Equation (3)) or the insertion of free cation (M+, such as Li+) into polymer skeleton (Equation (4)), which can also be reversed (Equations (5) and (6)). Meanwhile, the reduced state of PPy0A−M+ would also be further reduced through releasing the ions of A− and M+ (Equation (7)). Compared to the large size anions, the smallar cations tend to move in and out of the skeleton during the reduction and oxidation process, which can be attributed to the prone solvation of the small alkaline ions in water.[12] In other words, the anions are more likely to fix in PPy, thus leading to the cations transport through the poly mer framework.[20]

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Figure 4. a,c) The voltage and corresponding current density response curves of g-3D-PPy power generator upon exposure to 250 ppm of ethanol vapor. b,d) The voltage and corresponding current density response of g-3D-PPy to ethanol vapor with the concentration ranging from 0 to 250 ppm (dashed line: fit to a linear equation). e) The voltage and f ) current density outputs of the VaPG based on g-3D-PPy under various solvent vapors.

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Furthermore, electrochemical impedance spectroscopy (EIS) of PPy with the different RH range from 30% to 90% at room temperature revealed the intrinsic ionic-control conduc-tivity of PPy. As shown in Figure S13a (Supporting Informa-tion), the Nyquist plots show a remarkable decrease of resist-ance as increasing of the relative humidity (from 30% to 90% RH), indicating the ionic-led conductivity for PPy in relative wide humidity range,[6] which is further confirmed by the poor electronic conduction of PPy in either dry or wet environment (Figure S13b, Supporting Information).[48] More importantly, as a kind of hydrophilic dielectric materials, the 3D PPy with the interconnected macroporous structure is beneficial to the ionic transportation along with the layered nanostructures.[11]

A schematic illustration of the moisture-induced power generation mechanism for g-3D-PPy is shown in Figure 5 and Figure S14 (Supporting Information). With the accumulation of water molecules in g-3D-PPy, a regional solvation effect is enhanced, resulting in the broken of Li+ClO4

− bonds to release free Li+ and confined PPy+ClO4

−. Owing to the concen-tration gradient of Li+ induced by the gradient distribution of ClO4

− in g-3D-PPy, the small Li+ ion diffuses from high con-centration side to the low side once ionized by water molecules, thus leading to generate a strong induced potential along with the produced free electron movement of the external circuit (Figure 5a,b). While the Li+ ions come back and recombine with the PPy+ClO4

− groups after removing the water, resulting in a consequent reverse electron movement (Figure 5c). The electric signals of g-3D-PPy return to zero after the ambient water concentration falls to the initial state. The chemical potential-electric energy conversion process depends on the water adsorption and desorption behaviors, in which the water molecule accelerates the separation of Li+ and ClO4

−, leading to a difference electric potential between two sides of g-3D-PPy and converting the chemical potential energy to electric power. Based on above analysis, the high voltage output and power density output of g-3D-PPy can be mainly attributed to two aspects: (i) the high uniform gradient distribution of ClO4

− pro-motes the water molecules to dissociate plenty of free Li+ ions, thus generating a strong induced potential and current output; (2) the open porous 3D skeleton can greatly facilitate the dif-fusion of water molecules to offer large amounts of the disso-ciated charged ions, further accelerating the directed transport of the ionic charge carriers. In general, the unique designed

g-3D-PPy provides a new-type power generator for future self-powered vapor sensors.

Moreover, the above mechanism indicates that the vapors, which are able to dissociate LiClO4 into free Li+ and ClO4

− inducing the directional migration of free Li+, could drive the power genera-tion based on g-3D-PPy material. Consequently, most of organic solvents such as ethanol, methanol, and acetone,[44] and inorganic solvents like NH3⋅H2O and HCl⋅nH2O are suitable to act as the sources of stimulation, consistence with the corresponding elec-tric performances shown in Figure 4e,f. Besides, in order to fur-ther demonstrate the mechanism of the power generator based on the gradient of ClO4

− ions, the 3D PPy foams containing three dif-ferent concentrations of LiClO4 aqueous solution of 0.1, 0.5, and 1 mol L−1, were compressed successively to prepare the 3D PPy tablet with gradient steps of ClO4

− ions (Figure S15a, Supporting Information). The 3D PPy with gradient steps of ClO4

− ions pre-sents a similar voltage and current output behaviors to that of g-3D-PPy (Figure S15b,c, Supporting Information), indicating the gradient of ClO4

− ions within the 3D PPy foam induced the var-ious vapor-activated electric energy power generation.

3. Conclusions

In summary, we have demonstrated a high-performance vapor-activated power generator based on g-3D-PPy framework to convert the chemical potential energy induced by spontaneous organic/inorganic vapor diffusion to electric power. By electro-chemical establishing the anion-containing ions gradient in the PPy, the as-prepared g-3D-PPy provided a voltage output of ≈60 mV and a power density of ≈6.9 mW m−2 when exposed to the moisture environment. More interestingly, it also exhibited power generation behaviors upon exposure to most of organic or inorganic vapors, indicating the potential new type of self-powered vapor sensing applications. This work paves the way toward the exploration of conducting polymers in the future electric energy generation systems.

4. Experimental SectionPreparation of 3D V2O5 Foam: In a typical procedure, 0.36 g of V2O5

bulk was dissolved in 30 mL H2O containing 5 mL H2O2. The solution

Figure 5. A mechanism diagram of the VaPG based on g-3D-PPy. a) Schematic illustration of the gradient distribution of ClO4− in g-3D-PPy. b) Free Li+

ions were ionized by adsorbed water vapor and induced to establish a density gradient of Li+ ions. The free Li+ ions which diffused from the high to low density side induced the electrons orientation movement along external circuit. c) With the desorption process of water vapor, the recombination of Li+ ions and ClO4

− reduced the induced potential, resulting in reverse electrons flow in external circuit.

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was stirred and ultrasonicated for 10–20 min to form a clearly red solution, which was then sealed into a 50 mL Teflon autoclave and maintained at 190 °C for 18 h. Whereafter the autoclave was naturally cooled down to room temperature to get a bright yellow V2O5 gel. Finally, the 3D V2O5 foam was obtained after freeze-drying treatment.[23]

Preparation of 3D PPy Foam: The 3D V2O5 foam was directly sealed into a 50 mL Teflon autoclave containing 30 mL ethanol solvent with 10 vol% Py and treated at 190 °C for several hours to obtain 3D V2O5-Py foam. To fabricate the 3D V2O5-PPy foam, the as-prepared 3D V2O5-Py sample in contact with Pt foil was used as working electrode under a potential of 0.8 V in 0.2 m LiClO4 ethanol solution without additional Py monomer. The electropolymerization of Py was performed in a three-electrode electrochemical workstation (CHI660D instruments, Shanghai, China) with a polymerization charge quantity of 60 C,[19] in which a Pt sheet acted as counter electrode and Ag/AgCl as reference electrode. After that, the product was successively washed with 0.1 m HCl and water for several times.

Construction of 3D PPy Foam with Gradient of ClO4− Ions: The 3D PPy

foam containing 1 m LiClO4 aqueous solution was sandwiched by two gold electrodes in an enclosed environment under an applied voltage of 3 V for 500 s (Figure S3, Supporting Information). After EeA process, the as-prepared g-3D-PPy were in situ shorted to eliminate the capacitance effect where the circuit parameters were set as voltage = 0 V and time = 1000 s. Finally, the g-3D-PPy was was obtained after freeze-drying treatment.

Measurement of the Power Generation Process: The electric-related measurements including parameter setting were performed according to our previous reported papers.[10,11] Briefly, the sample was first sandwiched in two gold electrodes with vents and connected to a test circuit in a 20 mL enclosed container (Figures S6 and S11, Supporting Information). Moisture or other vapor carried by nitrogen and dry nitrogen was used to tune the RH. The voltage and current signals were recorded in real time using a Keithley 2400 multi-meter, which was controlled by a LabView-based data acquisition system.

Characterization: The morphology of the samples was examined with a scanning electron microscope (JSM-7001F) and EDS data were collected on a TecnaiG2 20ST (T20) at an acceleration voltage of 120 kV. XPS data were recorded on an ESCALAB 250 photoelectron spectrometer (Themo Fisher Scientific) with Al Kα (1486.6 eV). Raman spectra were conducted under ambient condition using a Renishawmicro Raman spectroscopy system with a laser at 532 nm. The X-ray diffraction patterns were recorded with a Bruker D8-Advance X-ray powder diffractometer, and Cu Kα was used as the radiation source (λ = 1.54 Å). BET specific surface area was determined by nitrogen adsorption−desorption isotherm measurements at 77 K (NOVA 2200e).

Supporting InformationSupporting Information is available from the Wiley Online Library or from the author.

AcknowledgementsThis work was supported by the National Basic Research Program of China (Grant No. 2011CB013000), the National Natural Science Foundation of China (Grant Nos. 21325415, 51673026 and 21575160), Beijing Natural Science Foundation (Grant No. 2164070, 2152028), the Fundamental Research Funds for the Central Universities, the Research Funds of Renmin University of China (Grant No. 14XNLQ04), and the Excellent Young Scholars Research Fund of Beijing Institute of Technology.

Received: August 13, 2016Revised: September 21, 2016

Published online:

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